
Class. 
Book. 



<Q&?# 



.. 



Copyright^?- 



COPYRIGHT DEPOSIT; 



Electricity 
Explained 



BY 

J. CALVIN S. TOMPKINS 




COCHRANE PUBLISHING CO. 

TRIBUNE BUILDING, NEW YORK 
1909 









Copyright, 1909, 

BY 

COCHRANE PUBLISHING CO. 

248227 . 



CONTENTS. 



CHAPTER 



PAGE 



I. Detecting Electricity with a Telephone Re- 



II. 


ttivti ........ 

Invisible Lines of Force 


X 

8 


III. 


Kinds of Currents Produced by Electricity 


13 


IV. 


The Alternating Current and the Pulsating 






Current . . 


21 


V. 


The Direct Current 


29 


VI. 


The Undulating Current 


34 


VII. 


The Oscillating Current 


38 


VIII. 


Wireless Telegraphy 


43 


IX. 


Wireless Telephony 


56 




Appendix of Formulae 


64 



Electricity Explained. 

CHAPTER I. 

DETECTING ELECTRICITY WITH A TELEPHONE RECEIVER. 

WE do not know what electricity is any more 
than we know what is lightning. Many 
theories have been advanced as to what men 
think this peculiar force is, but as yet we cannot say 
that we can define it. And yet we can see it and hear 
it. And we also can hear lightning. You say we 
can hear only the explosive discharge of lightning. 
Yes; but we can hear lightning itself, the moment it 
flashes, even though it is far from us. How? Sim- 
ply by connecting a telephone receiver between the 
gas pipe and the water pipe in the house. Or if you 
live in the country where there is no gas pipe, put one 
post of a telephone receiver in connection with the 
drainage pipe in the kitchen and connect the other 
post of the receiver with the ground by means of a 
water pipe, or the kitchen pump. 

Whenever you see lightning flashes, foretelling an 
approaching electric storm, put the receiver to your 
ear and listen, and you will hear the lightning bizz-zz 
as it discharges. This is static electricity discharging 
in the atmosphere. Now static electricity is elec- 

(i) 



2 ELECTRICITY EXPLAINED 

tricity at rest, which, when disturbed, discharges with 
considerable force. And force, as an electrical 
term, means volts. The atmosphere is full of 
this static electricity, and when it becomes heavy 
with it, or, in other words, when the atmosphere is 
fully charged with electricity, it breaks loose in se- 
vere discharges, which often prove fatal to destructible 
things within its path. 

There is not so much of this static electricity in the 
atmosphere in winter as in summer, but there is a 
great deal of it in the earth, and we can feel it and 
see it and hear it on the coldest day by walking briskly 
over a heavy carpet, and stopping quickly by a gas 
pipe or other grounded pipe, and placing our finger 
or hand near the pipe. A spark will fly from the 
hand to the metal pipe, and the positive electricity 
which we have picked up and stored temporarily in 
our body will jump back to earth, which is always 
negatively charged, with force (voltage) enough to 
produce a spark. And it takes twenty thousand volts 
to jump across space. Thus we see that there is sur- 
rounding us a power capable of producing great 
force. And yet we cannot harness it because it is 
instantaneous. With a single flash hundreds of thou- 
sands of volts are discharged between the clouds and 
the earth, and are lost. The average voltage in a 
flash of lightning is calculated to be about fifty million 
volts. 

Many interesting experiments can be made with our 
telephone receiver. In ordinary weather, when we 
listen to it, as it is connected between two grounded 



ELECTRICITY EXPLAINED 3 

pipes, we can hear very feebly a singing sound as of 
machinery. It is induction from the dynamo elec- 
tricity which feeds the town. The secondary winding 
of an alternating-current transformer is generally 
grounded, and if the town is equipped with trolley 
cars we will hear through our receivers a singing 
noise, sometimes loud and sometimes faint, as more or 
less cars take up the power into their motors. 

If we put a box-kite up in the air, with thin bare 
copper wire wound around the frame, or run an in- 
sulated copper wire up the side of a house and con- 
nect it with a number of bare copper wires hung from 



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MORSE CODE. 

the top of a flagpole, so as to catch the electric waves, 
and connect the end of this aerial with a very sensi- 
tive telephone receiver, and have this receiver also 
connected to the ground, we will hear wireless tele- 
graph stations communicating with each other; and 
if we learn the Morse code and the Wireless Conti- 
nental Telegraph code we may be able to read their 
messages. 

In order to catch distant messages from the air, 



4 ELECTRICITY EXPLAINED 

however, the aerial must reach five hundred feet 
above sea level, unless we employ a wireless detector 
with our receiver, the construction and use of which 
we will learn in another chapter. Also we must em- 
ploy a tuning coil to cut out more than one message 
at a time. Without a tuning system (which will be 
described later) we would hear the dots and dashes 
of several messages at the same time, and we could 
not translate them into words. 



ABCDEFGH 

fjt» «»•• •»••• ••• • ••«• «»•»• »••• 

I J K L M N P 

•* ••■»■• •»••* •••• ma mm ^ m mmmmmm mmt 

QRSTUVWX 

»■»•■• • •• ••• • ••• •••mm . <o« mmtmrnm 

V Z & . 12 3 4 

** mmm .•■"•• • ••* «• •• — •mmmm . w* l —mmm mmmmmm 

X -L J • »•,! ?' 

WIRELESS CONTINENTAL TELEGRAPH CODE. 

A lightning-rod on a country house will serve the 
purpose of an aerial if cut off from the ground and a 
wireless detector inserted between the two severed 
ends. But it must be well insulated from the house, 
as all lightning-rods should be, or the electric waves 
will jump to earth without going through the receiver. 
Electricity always takes the shortest path it can find 
to travel on, and this is most peculiar about a jumping 
spark. 

Now let us see what a telephone receiver is, and how 
it is made. When Alexander Graham Bell first in- 
vented the telephone, in the spring of 1876, he had a 



ELECTRICITY EXPLAINED 5 

coil of wire wound around an iron core. He placed 
two of these coils at a distance, connecting one end 
of each coil together and the other free ends he con- 
nected with a battery, one to the negative pole and 
the other to the positive. In front of each of these 
magnets was placed a tin plate, close to the magnet 
cores, but not touching them, and when Mr. Bell 
tapped on one of these plates, so that it pressed against 
the magnet core, the battery current was modulated 
and the distant magnet core would attract its tin plate. 
And when he talked loudly in front of one of these tin 
diaphragms, the sound waves pushed it varyingly to- 
ward the magnet, and accordingly the distant magnet 
pulled toward itself its tin diaphragm. And when 
these magnets were each inclosed in a case of rubber 
the action was intensified. So upon this principle the 
telephone receiver was made. 

When we unscrew the cap of the receiver which we 
use we find that the tin diaphragm sticks to the mag- 
nets, even when there is no battery connected with 
the magnet winding. Mr. Bell found that when the 
magnet cores were made of steel, permanently mag- 
netized, his telephones worked more successfully than 
with soft iron cores, and that one receiver could be 
directly connected to the other without any electric 
current in the circuit, and when he talked into one, 
the distant diaphragm would vibrate accordingly. 
Thus he gave us the first telephone. 

In another chapter we will learn about these lines 
of force which are invisible but which encircle all 
magnets, extending out beyond the end. And since 



6 ELECTRICITY EXPLAINED 

these lines of force do extend beyond the ends of 
magnets we can see how that, when a diaphragm vi- 
brates in front of a permanent magnet core, it dis- 
turbs these lines of force and changes their direction. 
This induces a varying current of electricity in the 
magnet of which the steel core forms a part, and this 
varying current, as it circulates around the distant 
magnet, increases and decreases the magnetism of the 
distant steel core, and the diaphragm which is in 
front of this permanently magnetized core vibrates in 
unison with the other transmitting diaphragm. So 
if we take two telephone receivers and connect them 
together with two wires we will have a complete 
telephone line, providing our receiver magnets are per- 
manently magnetized. Because of the loss of current 
in the connecting wires, however, and the stiffness of 
the receiver diaphragms, this is not a practical way 
to telephone over a distance exceeding two hundred 
feet. 

Now about the magnets in the telephone receiver 
to which we found the diaphragm sticking. Most re- 
ceivers have two magnets connected like the two mag- 
nets in an electric bell — in series. Such receivers are 
bipolar. The cores of these magnets appear to be 
permanently magnetized, but they are soft iron cores 
and we know that only steel can be permanently mag- 
netized. Then they must be connected with steel mag- 
nets, and upon examination we find in our receiver a 
horseshoe steel magnet the north-poled end of which 
connects with one magnet core and the south-poled 
end of which connects with the other magnet core. 



ELECTRICITY EXPLAINED 7 

Thus we see that the quickest varying currents of 
electricity, positive and negative, will vibrate the dia- 
phragm of a telephone receiver, and if we connect 
each post to separate ground connections, the slight- 
est disturbed current may be heard as it surges back 
and forth through the earth. 



ELECTRICITY EXPLAINED 



CHAPTER II. 

INVISIBLE LINES OF FORCE. 

Every one knows what a horseshoe magnet is, but 
not every one knows that one of the greatest princi- 
ples of applied electricity lies in the construction of 
such a magnet. Without a permanent magnet in the 
form of a steel needle, the ships of the sea would not 
venture to sail beyond the horizon, for there is a draw- 
ing magnetism of the earth, the composition of which 
has not yet been discovered, near the North Pole, 
which attracts this needle of the mariner's compass. 
When we look at the markings of a compass and 
see the needle pointing to the letter N, we should not 
conclude that this is the north pole of the magnetic 
needle. The end which rests over the letter S is the 
north-poled end, because like poles of a magnet repel 
each other, and unlike poles attract one another. And 
if the earth's North Pole attracts the end at letter N, 
then this end of the needle must be of south polarity. 

A piece of iron is composed of molecules of iron 
ore, welded together in a mass, which when heated in 
a furnace will expand. The red-hot horseshoe that a 
blacksmith is hammering looks very much thicker 
when he takes it out of the furnace than it does 
when he has finished hammering it. And it is made 
steel by crowding these molecules together. So we see 



ELECTRICITY EXPLAINED 9 

that even while in a solid mass these molecules can be 
made to change their shape. 

If we wind a coil of wire, insulated by a covering, 
around a bar of soft iron and pass a battery current 
of electricity through this wire, we will make a mag- 
net of the iron bar which will attract iron. But the 
moment we disconnect the battery current the bar of 
iron is no longer a magnet. Why? In passing an 
electric current through the wire wound around the 
iron we forced the molecules to take definite shape 
in the iron bar. But in soft iron they fall back again 
into irregular shapes when the current is cut off. If 
we try the same experiment with a bar of steel, when 
we break the electric circuit, and take the coil of wire 
off the bar we still find it has the properties of mag- 
netism, and will attract iron substances smaller and 
lighter than itself. The fact is that the molecules have 
been so crowded together that there is no room for 
them to fly back into irregular shapes after they have 
been magnetized. Steel is not elastic like soft iron, 
and will break before it will bend. It is true that the 
molecules do try to fall back irreguarly, and succeed 
in so doing after some years. That is why permanent 
magnets sometimes have to be remagnetized. Now to 
understand how these molecules act we may resort 
to a simple experiment. 

If we sprinkle some iron filings on a paper and un- 
derneath this hold a bar of steel, permanently mag- 
netized, we will see the filings try to arrange them- 
selves in form. And by tapping the edge of the paper, 
to aid the iron dust in jumping about, if the magnet 



io ELECTRICITY EXPLAINED 

is held lengthwise underneath, the form will become 
definitely symmetrical, arranging as shown by the 
dotted lines in Figure I. These regular lines, which 
we trace in the iron dust, bulge out from the center 
and stretch from one pole of the magnet to the other, 
remaining thickest around the two ends. These repre- 
sent our invisible lines of force. And while there is 
magnetism in any metal capable of retaining it, there 
will be these invisible lines of force surrounding that 
metal. One end of the magnet will be the north pole 
and the other end will be the south pole. And if the 
magnet is bent in the shape of a horseshoe, the lines 
of force will be strongest between the two ends. 



ji'V/'V';--- 



*x S v s ^ 

k&a&M 







FIGURE I. 

If we have a very powerful horseshoe magnet we 
will find it difficult to pass a plate of copper between 
the two poles, which plate would cut these lines of 
force. And with such a magnet we could blow out 
an electric arc light or any electric draw spark, 



ELECTRICITY EXPLAINED n 

which would break sooner than cut the lines of force. 
By snapping two live electric wires together between 
the two poles of such a magnet draw sparks will go 
out with an explosive report. The armature of a 
generator cuts these lines of force and produces a cur- 
rent of electricity in the armature coils in so doing. 
We shall learn of this later, but before we leave this 
subject we must know how to determine which is the 
north pole, and which is the south pole of an electric 
current. 

When we pass a direct current of electricity through 
a coil of insulated wire wrapped around a bar of iron 
we must have a positive flow of current and a nega- 
tive. The positive flow will magnetize one end of the 
bar, the center will be neutral, and the outflowing 
negative current will magnetize the other end. And if 
we hold a compass needle near the magnet, the end of 
the coil receiving the positive current will draw toward 
itself that point of the needle which rests over the 
letter N. Therefore, since unlike poles attract each 
other, and since the point of the needle resting over N 
is of south polarity, the end of a coil of wire receiving 
the positive flow of current must be the north pole and 
the end around which the negative current flows out 
must be the south pole. 

Now, when we know that electric current deflects 
a compass needle it is very easy to understand how 
meters are made to measure the current. Take a good 
compass and wrap around its diameter, from letter 
N to letter S, a few turns of fine wire. Pass a bat- 
tery current through this and the needle jumps at 



12 ELECTRICITY EXPLAINED 

right angles to the wires. Insert a slight resistance 
in the circuit, and the needle will jump in a direction 
less than a right angle. And insert a high resistance 
in the circuit and the compass needle will hardly move. 
With a disc chart for the face of the compass we can 
thus measure any resistance. 



ELECTRICITY EXPLAINED 13 



CHAPTER III. 

KINDS OF CURRENTS PRODUCED BY ELECTRICITY. 

There are five distinct kinds of currents which elec- 
tricity can be made to produce — the direct current, 
the alternating current, the pulsating current, the os- 
cillating current, and the undulating current. And it 
is interesting to follow the history of this science of 
electricity, and see how these currents were developed 
by experimental philosophers. 

The first principles of electricity were noticed in the 
metal called loadstone — earth's natural magnet. Then 
it was found by ancient experimenters that amber — a 
fossil gum found in certain geological formations — 
when rubbed by the skin of a cat or a piece of flannel, 
became magnetized, and was capable of attracting light 
bodies, such as pieces of silk and paper and feathers. 
This all happened six hundred years before the Chris- 
tian era. The Greek word for amber is elektron, and 
from this word the term electricity was derived. 

About the middle of the seventeenth century a 
German philosopher, Otto Von Guericke, invented an 
electric machine. He had found that glass could be 
made magnetic by friction as well as amber. By his 
machine he automatically rubbed a piece of revolving 
glass and then drew sparks from it with his hands. 
This gave static discharges, which oscillated to and 



14 ELECTRICITY EXPLAINED 

from separate posts on the machine when the glass 
was revolved. And to-day we call such a current 
an oscillating current. This electric machine was im- 
proved upon by an Italian experimenter, Aloisio Gal- 
vani, who one day, while using it, caused some sparks 
to jump to a frog's leg, and when he did so he noticed 
the muscles contracted with every spark. After this 
he found that without the aid of any electricity from 
his machine, when he placed two dissimilar pieces of 
metal at a distance on the muscle of a frog's leg, just 
such convulsions occurred. 

This experiment was followed by Alessandro Comte 
Volta, an Italian philosopher, who found that by plac- 
ing a piece of zinc on one part of an animal's flesh and 
a piece of silver on a distant part of the animal, and 
connecting the two by a metallic arc, the muscles of the 
animal would contract and stay so until the half ring 
of metal was taken away. It was then clear that when 
two such pieces of metal were laid on a wet surface 
a slight electric current would be produced. 

This was successfully tried by placing the zinc and 
the silver near together in a jar of salt water. Here 
then was another current, the direct current. And 
to this day the direct current is mostly produced 
chemically. For this reason it is one of the most ex- 
pensive currents produced, and is not used where other 
currents will do the work. But there are cases where 
necessity demands it. For instance, the telegraph and 
also the telephone are dependent upon the direct cur- 
rent. With an alternating current the telegraph relay 
would rattle like a loud buzzer every time the distant 



ELECTRICITY EXPLAINED 15 

operator depressed his transmitting key in sending 
messages. And if we were to talk over the telephone 
wires with an alternating current, our voices would 
quiver. 

But it is well to note right here that at the telephone 
central offices there are two and sometimes three cur- 
rents used — the direct current to talk with, and the 
alternating current, or for party wires the pulsating 
current, for central to ring the subscriber's bell. This 
is because the most serviceable bell for telephone sig- 
naling is the polarized bell which is used in connection 
with a condenser. In another chapter we will learn of 
what material condenser is made and to what use 
it is put. 

The alternating current already mentioned is pro- 
duced by machinery. From the word we know that 
this current must alternate, going backward and for- 
ward, as it does^ This is the cheapest commercial 
electric current produced, and by the aid of step-down 
or step-up transformers can be carried a great dis- 
tance and increased or decreased in voltage at will. 
Alternating current is measured by its voltage. There 
is not much amperage to it. When the alternations 
are very rapid it will heat carbon filaments in incan- 
descent lamps, and all other high resistances while hot. 
When used with a condenser in circuit the alternating 
current will surge back and forth through any high 
resistance, such as a telephone bell, which has a re- 
sistance of one thousand ohms, and without heating 
will pull and release the armature alternately, causing 
the hammer to strike both bell gongs. 



i6 



ELECTRICITY EXPLAINED 



If we take a horseshoe permanent magnet and re- 
volve between its ends a coil of wire we will produce 
in this coil, noticeable at its two free ends, an alternat- 
ing wave every time the coil is revolved. There are 
invisible lines of force between the two ends, or north 
and south poles of the permanent magnet, and as the 
coil of wire is revolved between these poles, it keeps 
cutting these lines of force. The alternating wave is 
always represented as in Figure II. And when half 
such an alternating wave is collected from a specially 
constructed dynamo, we get another current — the pul- 
sating current. We shall learn more about this cur- 
rent later, which is used to peculiar advantage in the 
party-line telephone system of selective ringing. 




FIGURE II. 



A current very much like the pulsating current, but 
which requires no dynamo to produce it, is called the 
undulating current. If we connect an electric bell or 



ELECTRICITY EXPLAINED 17 

buzzer with two or three battery cells and then place 
a finger of one of our hands on the iron frame and an- 
other finger on the adjustable contact screw, we will 
get a shock. The current we thus get is an undulating 
current. It feels like an alternating current, but it 
differs from that in its manner of production and in 
its varying strength. It is not as constant as an alter- 
nating current, and requires a close adjustment of the 
bell armature to keep it from varying. But it has re- 
cently been found that by including powerful in- 
ductive magnets wound with heavy wire, in circuit be- 
tween the battery cells and the buzzer, the voltage can 
be greatly increased, so that a few dry cells battery and 
a small electric buzzer connected with a large in- 
ductive coil of wire in circuit will give an undulating 
current between the frame of the buzzer and the ad- 
justable contact screw powerful enough to light several 
incandescent lamps. This is a simple experiment 
which any one can make, and from which much can be 
learned. 

Having a buzzer equipped with an adjusting spring 
to loosen or tighten the armature we can increase or 
decrease this current at will, thus doing away with a 
rheostat or any other costly means of control. Then, 
too, if we connect a telephone receiver between the 
contact screw and the frame of the buzzer, and hold 
our receiver in a jar or glass tumbler, we will produce 
a musical note. Thus we can understand how the 
telharmonium produces electrical music, only that, in- 
stead of using an undulating current, Dr. Cahill, its 
inventor, makes use of the alternating current. There 



18 ELECTRICITY EXPLAINED 

are one hundred and forty-eight dynamos running in 
the Telharmonic Hall in New York City, all at dif- 
ferent speed, and with a keyboard composed of elec- 
trical switches. The current is controlled and sent out 
over two wires terminating in a telephone receiver. 

Another interesting experiment we can try with our 
receiver and buzzer is to place the hand tightly over 
the cap of the telephone receiver, and quickly releasing 
it as quickly again clap the hand over the cap. With 
practice we can make our receiver talk like a child 
when it first attempts to say "papa" and "mamma/' 
With a suitable set of mouths capable of being opened 
and closed at will, to produce different vowel sounds 
and words, we could make a very good electrically 
talking machine. But the cost would far exceed that 
of the phonograph, which does more perfect work than 
an undulating current would do. 




FIGURE III. 



We have given some attention to the oscillating cur- 
rent, and found it was first produced by an electric 
machine. .But it is more easily and much more 
cheaply produced by the induction coil. An oscillat- 



ELECTRICITY EXPLAINED 19 

ing wave is shown in Figure III, and is one which 
dies away in space, thus differing from an alternating 
current wave. 

Electro-magnetic induction was the result of Michael 
Farady's experiments. Farady, an English scientist, 
found that when he inserted a steel rod, permanently 
magnetized, in a coil of wire the two ends of which 
were connected to an electroscope, a current of elec- 
tricity was produced in the coil, and by quickly moving 
the permanent magnet to and fro in the coil intermit- 
tent currents were produced. Then he substituted a 
coil of heavy wire wound on a soft iron bar and con- 
necting with a battery, in place of the permanent 
magnet, and found that by inserting this in the coil 
of wire these intermittent currents were intensified. 
Afterward he wound the second coil over the heavy 
wire coil and passed the positive current through this 
primary coil to an armature made of iron and held 
away from the core by a spring, and through a con- 
tact screw against which this armature rested, to the 
negative side of the battery current. When the cir- 
cuit was thus closed the current flowing through the 
magnet would draw the iron armature to its core and 
away from the contact screw, so breaking the circuit 
and demagnetizing the core, allowing the spring to 
pull back the armature against the contact screw 
again, thus closing the circuit and repeating the opera- 
tion. This produced intermittent currents in the sec- 
ondary coil, which contracted the muscles when the 
two ends of this coil were held in the hand, and was 
called the faradic current, after its inventor. 



20 ELECTRICITY EXPLAINED 

But later it was found that when the secondary coil 
was composed of several miles of very thin insulated 
copper wire wound around, and well insulated from, 
the primary coil, making a very high resistance, sparks 
would fly across the coil ends and a current of over 
thirty thousand volts was produced. To-day we call 
this an oscillating current, and we find it high potential 
because of its enormous voltage, and high frequency 
because of its rapid motion — the oscillations moving 
at the rate of two hundred and thirty millions per sec- 
ond. This is the current which gives us wireless 
telegraphy and the X-rays. In the following chapters 
these five different currents will be separately treated, 
and we shall find how each is produced. 

There is another way of producing a slight electric 
current, however, and that is, by extreme variations of 
temperature. This is called the thermo-electric cur- 
rent, but has never been put to practical use because 
it is too weak. Such a current may be produced by 
running copper and iron bars alternately through holes 
drilled in the sides of a box, and joining their ends so 
that they are connected in series, and then forcing an 
extreme cold blast of temperature against one side of 
the box and applying heat to the other ends. The 
current will be derived from the free end of the first 
wire or bar placed in the box and the last bar, and 
if an electric bell is connected with these two ends 
the current will be strong enough to ring it. 



ELECTRICITY EXPLAINED 



21 



CHAPTER IV. 

THE ALTERNATING CURRENT AND THE PULSATING 
CURRENT. 

We have found how a coil of wire, when revolved 

90* 




<-Z70 9 



FIGURE IV. 



between the north pole and the south pole of a horse- 
shoe magnet, cuts the lines of force and creates an 
alternating current wave. A dynamo is built upon this 



n ffiliis 



FIGURE V. 



principle, and has been made a practical producer of 
electricity by Thomas Edison and by S. P. Thompson. 
In Thompson's own work : "The dynamo electric ma- 



22 



ELECTRICITY EXPLAINED 



chine is a machine for converting energy in the form 
of mechanical power into energy in the form of elec- 
tric currents, or vice versa, by the operation of set- 
ting conductors (usually in the form of coils of cop- 
per wire) to rotate in a magnetic field." 




FIGURE VI. 



In Fig. IV we have an imaginary view of an 
alternating current wave with a center line, marked 
off in degrees. And in Figure V we see the im- 
aginary lines of force stretching between the north 







FIGURE VII. 



pole of a magnet and the south pole of another mag- 
net. Figure VI. shows a coil of thin wire inserted 
between these two permanent magnets, exactly in line 



ELECTRICITY EXPLAINED 23 

with these lines of force. When this coil of wire is 
moved thirty degrees of a revolution it has begun to 
cut these lines, and when it has moved ninety de- 
grees it has cut the greatest number of lines of force 
within its field, and the coil of wire is perpendicular to 
these lines, as shown in Figure VII. Then as the coil 
is kept revolving it comes again in line with these 
lines as it reaches one hundred and eighty degrees, 
and when it has been revolved two hundred and sev- 
enty degrees our coil again cuts the greatest number 
of these invisible lines in the field, and finally comes 
back to its original position, after it has been revolved 
three hundred and sixty degrees of a circle. During 
each revolution, then, we see the induced voltage rises 
from zero to maximum, then decreases, then reverses, 
then rises to maximum in the opposite direction, and 
decreases to zero again. If we connect a telephone 
receiver between the two free ends of this coil of 
wire, as shown in the drawings, we can detect the 
current generated as we revolve the coil. 

When we wind our coil on an iron core, through 
the center of which has been drilled a hole with a 
tightly fitted axle inserted therein, perpendicular to 
the length of the core, and upon this axle place a 
fiber insulating spool near one end, and over this 
spool fit a brass collar, we can make an armature by 
connecting one end of the coil to the brass collar and 
the other end to the iron core. Upon this principle is 
the so-called magneto made, which has one metal 
brush rubbing against the collar and another such 
brush rubbing against the axle of the core. When 



24 ELECTRICITY EXPLAINED 

such a machine is made to run swiftly by power we 
can pick off these alternating current waves from the 
two brushes, and produce a voltage of electricity. A 
volt generated thus is equal to the armature coil cut- 
ting one hundred million lines of force per second. 
And the number of lines of force in the field of a 
horseshoe magnet varies in proportion to the strength 
of the magnet. So we see the stronger the field of a 
dynamo and the faster the armature is revolved within 
this field, the greater will be the voltage. 

But if our magneto is small we will hardly get more 




FIGURE VIII. 

than ten volts, so we must find out how the central 
supply station produces enough voltage to light the 
street lamps and houses. In such a station there are 
immense dynamos having many armature coils lapped 
around the core as shown in Figure VIII and con- 
nected to rings of metal, like our magneto collar, 



ELECTRICITY EXPLAINED 25 

placed side by side, but not touching, on the arma- 
ture shaft. Or the armature coils are wound near to- 
gether on a soft iron ring core, as shown in Figure 
IX. So we see, when such an armature is revolved 
within the field of magnetism of many magnets, al- 
ternating currents can be taken from brushes of metal 




FIGURE IX. 

bearing upon each armature ring. There are gen- 
erally half as many stationary coils of heavy wire en- 
circling the armature coils, which are mounted upon 
the frame of the generator and connected as shown 
in Figure X, which take the place of permanent mag- 
net. Figure VI shows a coil of thin wire inserted 
But to feed these field magnets with a battery current 
would be very costly, so the dynamo is made to ex- 
cite its own field by means of a commutator which 



26 



ELECTRICITY EXPLAINED 



picks out from these armature rings a direct current 
to feed the field magnets. This is done in three dif- 
ferent ways. 

( i ) With the series winding the field coils are con- 
nected in series with the main circuit, which leaves the 
dynamo at the brushes. The alternating current and 
the commutator derived direct current are both col- 
lecting voltage from the same armature coils, and 
therefore the field is said to be in series with these 
coils. 




FIGURE X. 

(2) With the shunt winding the field coils are con- 
nected to the commutator brushes and the main cir- 
cuit to the ring-type collectors. Both of these kinds of 
brushes are collecting currents from the same coils, it 
must be remembered; therefore the field coils are 
said to be in shunt with the brushes. 

(3) With the compound winding — a combination 
of the series and shunt — the field magnets are wound 
with two sizes of wire, side by side, the coils of 
heavy wire being connected in series with the main cir- 
cuit, and the coils of thin wire being connected with 
the brushes. 



ELECTRICITY EXPLAINED 27 

So far we have considered only the single phase al- 
ternating current, which occurs when such currents 
rise and fall at the same time. When they are out of 
step — that is, when one current lags behind the other 
— they are polyphase. A two-phase current* is pro- 
duced by having two independent wires wound side by 
side, for each armature coil, the ends of each such 
winding connecting to a separate pair of collector 
rings. This gives us the pulsating current, in which 
the direction of the current is always the same, but 
which current rises to a maximum and falls to zero 
twice during each revolution. The pulsating cur- 
rent wave is shown diagrammatically by a drawing. 




PULSATING CURRENT WAVE. 

A triphase current wave is made of three loops where 
the two-phase current has but two, because the tri- 
phase current rises to a maximum and falls to zero 
three times during each revolution. In carrying these 
polyphase currents for some distance from the dy- 
namo, each phase current is made to pass through a 
two-wire cable, and two, three, or more cables con- 
vert the whole current to step-up transformers, or 
to transformers that step down the current. 

The motors used to run machinery are different 
from the direct current motors. The direct current 



28 ELECTRICITY EXPLAINED 

motor has field and armature connected in series, the 
polarity of one first attracting, then repelling that of 
the other, while the alternating current motors have 
their coil polarity constantly changed by the alter- 
nating currents sent through them. 

If one is in doubt as to what kind of current his 
house is lighted with, let him take a powerful magnet 
and hold it against an electric lamp. If the current 
is alternating the lamp filament will sway back and 
forth. If the current is direct the magnet will pull 
the filament toward itself. 



ELECTRICITY EXPLAINED 29 



CHAPTER V. 

THE DIRECT CURRENT. 

We have found how the direct current was dis- 
covered by Galvini, and improved by Volto when he 
placed a piece of silver and a piece of zinc in a glass 
of salt water and produced a current of electricity. 
This was first called the galvanic current, and in the 
study of electrotherapeutics it has always been so 
called. 

Many experimenters have developed the chemical 
production of the direct current of electricity since 
then, and to-day we have more than twenty-five liquid 
cells of different makes, and about one hundred dif- 
ferent kinds of dry cells. The highest voltage ob- 
tained from either kind of these primary batteries that 
are now in use is not more than two and one-half volts 
per cell, with an internal resistance of about two ohms. 
But the primary battery is still being improved, and it 
is claimed that some of the latest of these cells are 
powerful enough to light several electric lamps, when 
attached to the feed wires of a house wired for elec- 
tric lights. When this becomes practical it will take 
the place of the central station system for house light- 
ing. 

Carbon is universally used now, instead of silver, 
for the negative plate of a primary cell; it is cheaper 



30 



ELECTRICITY EXPLAINED 



and more serviceable, but we cannot find anything 
more serviceable than zinc for the position plate. We 
call the carbon the position pole of a cell, but it is the 
negative plate, because the current flows from the 
zinc to the carbon of the cell. In the fluid cell the 
zincs will crystallize because oxygen bubbles collect 
upon the surface of the zinc. So here is a way to de- 
termine the negative side of a battery circuit. When 
the two ends of the line connecting with a battery are 
placed in a glass of water, bubbles of oxygen will 
collect on the negative wire. 

The storage battery is another convenient means for 
producing a direct current of electricity, but this is a 
secondary battery in that it is dependent upon ma- 
chinery to charge it. There are many different kinds 
of storage batteries, large and small, all of which give 
a low voltage but a very high amperage. 



v 



M 



FIGURE XI. 



The machinery made to produce a direct current 
with which to charge storage batteries is a direct cur- 
rent generator. This is made like the dynamo except 
that the armature coils are connected to half rings 
instead of ring collars, the two ends of each coil 



ELECTRICITY EXPLAINED 31 

being connected one to one segment and the other 
to the other segment which is separated from it. Fig- 
ure XI shows a brass tube cut in two segments. Re- 
ferring to the preceding chapter, if we remove the 
brass collar from the insulating armature spool of our 
magneto, and fasten these segments thereon, and con- 
nect the coil, one end to each segment, we will have 
a commutator which will pick out a direct current of 
generated electricity. The direct current is continu- 
ous only in its direction. There may be a number of 
brushes or there may be only two. 

Many direct current distributing plants have storage 
battery rooms as well as direct current generators. 
The storage batteries are charged during the daytime, 
when there is not a great output of current, and con- 
nected to the main circuit by a double throw switch 
at night and discharged. 

A storage battery has two plates, the simplest cell 
being made of two lead plates immersed in a jar of 
dilute sulphuric acid. When a direct current is passed 
through this cell the water in the acid is decomposed 
and the oxygen passes to the positive plate and unites 
with the metallic lead, forming a coating of red per- 
oxide of lead. The hydrogen is mostly set free from 
the negative plate, and only upon the discharging of 
the cell does it combine again with the oxygen. So 
a storage battery is a battery the chemical action of 
which is reversible. 

There is another battery cell used to produce a di- 
rect current of electricity, which, because of its con- 
stant action and its non-polarizing effects, has been 



32 ELECTRICITY EXPLAINED 

much used in telegraphy. This is the gravity cell. 
Two liquids, sulphuric acid and copper sulphate, are 
used, their difference in specific gravity serving to 
keep them separate. The copper plate is placed in 
the bottom of a jar and covered with crystals of 
copper sulphate and a solution of the same. Above, 
at the top of the jar, is a zinc plate. Sulphuric acid, 
being lighter in weight than copper sulphate, is then 
poured into the jar until it covers the zinc. The two 
solutions do not remain entirely separate, as the cop- 
per sulphate diffuses somewhat into the lighter acid. 
A gravity cell may be used as a storage battery by 
sending a direct current reversed into it, but is not 
practical for this purpose because of the diffusion of 
the two solutions, which sets up destructive local 
action. 

Before leaving the study of the direct current we 
might well learn of some uses to which it can be put. 
Because of its amperage its heating effects are most 
noticeable. An electric lamp is made to glow and give 
light by the heating of its carbon filament. An al- 
ternating current heats and gives light by crowding 
the molecules of any great resistance in its surging 
back and forth through them. So we see that re- 
sisted current produces heat, and now we can under- 
stand why electric heaters are made of coils of fine 
wire. 

There are several distinct kinds of resistance ma- 
terials which give us the tungsten lamp, the helium 
lamp, the tantalum lamp — all of which are named after 
the material of which the lamp filament is composed — 



ELECTRICITY EXPLAINED 33 

and the Nerst lamp, which is named after its Ger- 
man inventor, and is made of a mixture of rare earths, 
worked into a dough, then pressed through a die and 
baked. 



34 ELECTRICITY EXPLAINED 



CHAPTER VI. 

THE UNDULATING CURRENT. 

An undulating current is one which has vibratory 
properties. It is sometimes called an intermittent cur- 
rent because of its tendency to vary. It is something 
like a polyphase current in that it is lagging in na- 
ture. But an undulating current is produced only by 
a direct current of electricity flowing through a coil 
of wire wound over an iron core, arranged with a 
make-and-break armature at one end of this core, 
such as the hammer of an electric bell. 

If we connect one side of a battery to a bell binding- 
post and strap a telephone receiver between the other 
side of the battery and the other bellpost, we will 
hear through the receiver the make and break of the 
current as the armature vibrates. And if we put a 
piece of metal across the receiver binding posts the 
current flowing through the buzzer will be stronger. 
Then a resistance cuts down the current flow at the 
distant end of the circuit. And if we take hold of the 
bell frame with one hand and the adjustable contact 
screw with the other, we will feel a slight sensation of 
"shock," which will be strongest when the receiver is 
short circuited. If now we substitute a coil of very 
heavy wire, insulated and wound over a soft iron core, 
in place of the receiver, and then touch the bell frame 



ELECTRICITY EXPLAINED 



35 



and adjustable contact screw, we will not be able to 
hold fast, the undulating current will be so strong. 




UNDULATING CURRENT SYSTEM. 



When a great amount of insulated copper wire is 
wound over in an iron core, extra currents of self- 
induction will be produced. This we can prove by 
strapping out the bell at its binding posts, and taking 
off a wire at one side of the battery, making touch 
connections. There will be bright draw sparks at 
this point which if held near any explosive gas will 
ignite it. Upon this principle is based the ordinary 
electric gas-lighting system. With an inductive re- 
sistance of the fine wire there would not be much of a 
spark because the resistance of the wire would be too 
great. And even if the wire was the same size as that 



36 



ELECTRICITY EXPLAINED 



used to carry the battery current through the house, 
the current would be cut down. 

Now, Ohm's law is that the current measured in 
amperes is equal to the voltage divided by the ohmic 
resistance. Then the voltage is equal to the amperes 
multiplied by the ohms, and the ohmage is equal to 
the voltage divided by the amperage. If we have 
four volts and four amperes in our battery we would 
have to have an induction coil with a resistance of 
about one ohm, and if we use a small storage bat- 
tery with an electromotive force of two and two- 
fifths volts and eight amperes we would have to use 
an induction coil of heavier wire, with a resistance not 
greater than three-tenths of an ohm. So here we have 
a means of increasing the voltage of the undulating 



rt 



a 



WIRELESS TRANSMISSION WITH UNDULATING CURRENT. 

current with induction coils. And by connecting wires 
to the buzzer frame and the adjustable contact screw, 



ELECTRICITY EXPLAINED 37 

respectively, we can connect these wires to electric 
lamps and derive enough undulating current to light 
up a large house. Also this current can be used to 
transmit wireless messages over a short distance in- 
stead of employing the costly spark induction coil, 
by grounding one binding post of the bell. The 
messages can be received with a telephone receiver 
strapped across a water pipe and a gas pipe, or two 
different ground connections. In fact, if an ordinary 
doorbell is grounded, or any part of its wire circuit 
touches a grounded pipe in a house, if we listen with 
our receiver strapped across two grounded pipes, we 
will hear a faint undulating current when some one 
pushes the button that rings our doorbell. 

When a very fine wire is stretched for a distance 
of half a mile and an undulating current is sent 
through it, we can pick up the transmitted current in 
another wire half a mile away if this wire is like 
the transmitting wire, half a mile long and thin, pro- 
viding the two ends of the receiving wire are con- 
nected to the two posts of a telephone receiver. This 
scheme can be used for the transmission of wireless 
telegraph messages for any distance if that distance 
is not greater than the length of each wire. For this 
reason such a system is limited. Another chapter will 
tell us how wireless telegraphy is best produced, and 
what apparatus is used to produce it and to receive it. 
But first we must study the oscillating current which 
makes wireless practical. 



38 ELECTRICITY EXPLAINED 



CHAPTER VII. 

THE OSCILLATING CURRENT. 

An oscillation is an alternate ascent and decent of 
something with motion. When we suspend a weight 
on a string or a pendulum from a clock and allow it 
to swing back and forth we say it oscillates. And if 
we go to a lake of smooth water with two small stones 
and throw them both into the water at the same time, 
there will spread out from the place where each stone 
struck little waves which will intermingle and oscil- 
late, those made by one stone moving toward the 
center of oscillation of the other. Now, to produce 
electric oscillations we use one induction coil, the two 
ends of the secondary winding connecting with two 
separated brass balls which we call the oscillators. 
When we send an interrupted current through the 
primary coil we force a current in the opposite direc- 
tion into and through the secondary winding, which 
acts as a number of elastic rings, fitted tightly on a 
rod, would act if we were to push the end one and 
crowd all the rings toward the other end of the rod. 
The rings would twirl and turn inside out time and 
again in the operation. And so an interrupted elec- 
tric current pushes from one coil of wire into the 
other of the secondary winding with a force so strong 



ELECTRICITY EXPLAINED 39 

that a high frequency electric oscillating current jumps 
with a spark from one oscillator to the other. This 
spark is the visible intermingling of oscillating waves. 

As the waves spread out from all sides of each stone 
thrown into the water, so these electric waves spread 
out from all sides of each oscillator. We prove this 
by disconnecting the two brass balls and holding a 
finger near one oscillator, and seeing the hitherto in- 
visible electric waves visibly jump to the finger. So 
in wireless telegraphy, where the oscillating current is 
used, there are invisible waves vibrating against the 
ether in the atmosphere at the rate of two hundred 
and thirty millions per second. These waves were 
discovered by Doctor Hertz, who did much to demon- 
strate their properties, and they are frequently called 
Hertzian waves. They are longer than the waves of 
light, but do not reflect their heating properties, al- 
though they are hot enough to burn. Waves of light 
are visible, but Hertzian waves are so much longer 
that they are invisible except when we place our hand 
near an oscillator, which shortens them and renders 
them visible. 

Professor Ruhmkorff found in experimenting with 
induction coils that if much of a jump spark was de- 
sired an enormous amount of costly fine wire was re- 
quired for the helix, or winding, of the secondary 
coil. So to do away with this expense he connected a 
condenser between the armature and the contact screw 
of the primary coil. This greatly increased the length 
and thickness of the jump sparks at the oscillator and 
also diminished the brilliancy and destructible prop- 



4 o ELECTRICITY EXPLAINED 

erties of the make-and-break spark at the platinum 
pointed contact screw. 

The main feature of a condenser when connected in 
shunt across a circuit of electricity is to catch and 
absorb all unnecessary fluctuations. A leyden jar is 
the simplest form of a condenser, and is easily made 
with a glass tumbler, or wineglass, and two sheets of 
tinfoil. One sheet is pasted tightly to the inner sur- 
face of the glass, and the other sheet is likewise se- 
cured to the outside of the glass. And a condenser 
is made up of many layers of tinfoil separated by pa- 
per, each alternate sheet of tinfoil being connected 
together. If one set of tinfoil ends is connected to 
a wire and the other set of ends is connected to another 
wire, we will have an accumulator which will not 
short circuit an electric current but, rather, will hold 
a large amount of electricity on its comparatively small 
tinfoil surface. 

But not only is an oscillating current produced by 
an induction coil. It is frequently produced by an 
electric machine which is often called a static ma- 
chine. This in its simplest form consists of a round 
glass plate mounted on a wooden shaft furnished with 
a handle or pulley wheel for the purpose of rotation. 
Two wooden posts are mounted in line with the edge 
of the glass wheel, one of which holds a forked stick 
which fits around the edge of the wheel. On each 
tongue of this forked stick is fastened a rubber cushion 
covered with an amalgam of tin, zinc, and mercury, 
which rubs against the sides of the glass plate. These 
are connected to a metal hook fastened in the wood 



ELECTRICITY EXPLAINED 41 

post which forms the negative side of the machine. 
Mounted on the other post is a forked stick, the 
tongues of which carry metal teeth which point toward 
the glass wheel but do not touch it on either side. 
These are connected to a hook and form the positive 
pole of the machine. When the wheel is revolved the 
friction electricity produced jumps to the metal teeth, 
and if we stretch a wire from the positive hook and 
fasten it near the negative hook, with the latter con- 
nected by a wire to the ground, sparks will fly, de- 
noting that electric waves are oscillating from the two 
hooks. 

Now since we have seen how this oscillating cur- 
rent is produced, we shall learn to what uses it can 
be put. We have found that wireless telegraphy is 
dependent upon it. Professor Roentgen discovered 
that, with the aid of a glass vacuum tube invented by 
Crooke, with two platinum wires sealed therein, these 
Hertzian waves, when oscillating between the two 
platinum wires, emitted extra rays of light which were 
different from any other light rays, and he called them 
the "X-rays/' 

Everything placed in the path of these X-rays be- 
comes more or less transparent. Paper and wood no 
more screen these rays than does glass the rays of the 
sun. But bone and pieces of glass resist these rays, 
and metal completely hides them. In the science of 
medical surgery and electrotherapeutics, therefore, 
these peculiar rays play a most important part to 
diagnose enlargements and localize the presence of 
foreign substances in the human body. 



42 ELECTRICITY EXPLAINED 

When an inductor coil is used to produce X-rays 
the interrupter is frequently done away with and the 
primary coil connected direct to a generator produc- 
ing an alternating current. At least a three-inch 
spark is required to oscillate in a Crooke's tube to pro- 
duce these X-rays. This means a high potential cur- 
rent at the secondary of the induction coil of ninety 
thousand volts. 

By placing the hand in front of a Crooke's tube 
the bones can be seen fairly well, but if we look 
through a fluoroscope at our hand when in the path of 
X-rays very distinct outlines of the bones and even 
some of the muscles may be seen. A fluoroscope is a 
box-like screen with a piece of cardboard fitted in 
one end covered with crystals of tungstate of calcium. 

The art of photographing parts of the body is very 
simple with the use of the X-rays. A sensitized plate 
can be left in the plateholder and the hand placed upon 
it, and the oscillating current sent through a Crooke's 
tube in daylight, for any length of time. The X-rays 
will penetrate through the plateholder, and when the 
plate is developed in the usual way a good negative 
representation of the bones and muscles of the hand 
will be obtained. From this any number of printed 
pictures may be taken. 



ELECTRICITY EXPLAINED 43 



CHAPTER VIII. 

WIRELESS TELEGRAPHY. 

We have already learned how an induction coil is 
made, and how it produces Hertzian waves, and that 
these waves form an oscillating current of electricity. 
We have also found that these waves can be drawn off 
the oscillator of an induction coil with one's hand, 
proving that they spread out invisibly in the ether of 
the air, and are longer than waves of light. In fact 
daylight, and particularly sunlight, absorbs Hertzian 
waves to such an extent that they will travel two and 
one-half times farther at night than by day. And for 
this same reason, due to the ionization of the air by 
daylight, a clear day is less preferable than a foggy 
day for the transmission of wireless messages. 

In the year 1890 Branly, a Frenchman, discovered 
that when iron filings were placed in a tube sealed 
at each end by pieces of silver, and placed near an 
oscillating current, they packed together, and when a 
bell and battery were connected in series with the sil- 
ver ends of the tube, the bell rang, where before the 
filings offered too high a resistance to carry a battery 
current of electricity. This was about the time when 
theories of magnetism were being advanced, and the 
working of the molecules of iron was being demon- 
strated by the use of iron filings placed in a glass tube 



44 ELECTRICITY EXPLAINED 

and encircled by a coil of insulated wire. Branly 
called this a radio-conductor, but it was later called 
a coherer because of the packing or coherence of the 
filings. But when once the Hertzian waves caused 
this coherence, the filings would not decohere unless 
jarred apart. To signal without wires by ringing an 
electric bell, therefore, it was necessary to decohere 
these filings as fast as they cohered, and this was made 
possible by the work of a Russian named Popoff, who 
connected a relay and battery in series with the two 
silver ends of the metal filings tube. This relay, like 
all relays of to-day, was a magnet with an armature 
which touched a separate wire when drawn against 
the magnet core, and through this wire and the arma- 
ture a path was formed for another electric current. 
So where the relay was connected with a battery to 
the two ends of a coherer, and the Hertzian waves 
transmitted from a distance, this caused the metal fil- 
ings to cohere, the armature of the relay was drawn 
against the core and touched the separate wire. An 
electric bell and battery was connected in series with 
this wire and the relay armature, and was placed so 
the bell hammer would strike the glass tube and de- 
cohere the filings on its backward strokes after tap- 
ping the bell gong. Thus the bell could be operated 
from a distance and given signals at the operator's will. 
In 1895 Marconi of Italy connected one end of this 
coherer to the ground and the other end to a wire 
stretched high in the air, and, like Popoff, a relay 
and battery in circuit with the two ends of the coherer. 
Instead of the signal bell he substituted a Morse tele- 



ELECTRICITY EXPLAINED 



45 



graph register with a battery and tapper between the 
relay armature and wire contact. The tapper was 
like a bell armature and coils, and was operated in the 
same way as any electric bell. When the filings co- 
hered, because of the Hertzian waves, the relay and 
battery drew up the armature against the separate 




WIRELESS TRANSMITTER. 

wire, and the second local battery operated the Morse 
register and tapper. Marconi also grounded the os- 
cillator on one side of the transmitting coil and con- 
nected the other oscillator with an aerial wire. And 
instead of the silver rods in the ends of the coherer he 
inserted silver wires with but slight separation, and 
placed filings of nickel instead of iron dust in the glass 



4 6 



ELECTRICITY EXPLAINED 



tube. Then he found that if more than half the filings 
were nickel and the rest silver they would decohere 
more readily, especially if the air was exhausted from 
the glass tube. Two years later this inventor brought 
wireless telegraphy into commercial use. Then others 
who had been experimenting secretly with this art of 
wireless signaling began to demonstrate their appara- 
tus, until to-day we have a great variety of schemes 
and improvements in the art. 



v 



£ 



J-8%-4 



n 



W; 



m 



•H 



WIRELESS COHERER. 



In order to clearly understand what benefit is de- 
rived by the aerial of Marconi, which solved the prob- 
lem of long distance wireless transmission, we must 
consider the transmitter. We know that we can pick 



ELECTRICITY EXPLAINED 47 

off Hertzian waves from either oscillator of the sec- 
ondary coil with our hand. We ground one oscillator 
and connect the other to an aerial wire, from which 
we can pick off these waves. This is because they 
tend to flow toward the earth. Therefore, when the 
aerial is perfectly insulated from the ground and an- 
other aerial is stretched in the air at the receiving 
station, these waves jump to the receiving aerial and 
down to the ground. And if a resistance of metal 
filings is placed between the ground and the receiv- 
ing wire stretched in the air, we can detect with a 
telephone receiver when these waves push through the 
filings and pack them together. 

There are many different ways of receiving mes- 
sages, and this has been the larger field for improve- 
ment. The substitution of oxide of lead in place of 
iron or nickel filings in the coherer was found to anti- 
cohere, and when used with a gravity battery, the cur- 
rent from the battery flowing through such a coherer 
binds the oxide of lead into small strands of metal, 
which are broken by electric oscillations but immedi- 
ately resume their thread-like form when the oscilla- 
tions cease. However, the coherer is the only type 
of receiving detector which will ring an electric bell 
or vibrate a relay armature when connected with a 
local battery circuit. The anti-coherer is used with 
the telephone receiver, as it is much more sensitive 
than a relay. Because of this sensitiveness the re- 
ceiver has given rise to a number of different styles 
of liquid detectors, in which platinum wire is used, 
one platinum point being connected with the earth and 



4 8 



ELECTRICITY EXPLAINED 



one side of a telephone receiver and the other platinum 
point being connected with the other side of the re- 
ceiver and the aerial wire, and these platinum wires 
immersed in a liquid. 




LIQUID DETECTOR. 

A very good liquid detector can be made by break- 
ing the end off an old electric lamp, and destroying the 
filament with a hatpin, placing the lamp in a socket 
and inserting in the broken end enough nitric acid to 
cover the platinum points. Connect this socket to the 
ground and to the aerial wire respectively at the two 
binding posts, and then strap a telephone receiver 
across these two connections. The ether of the at- 



ELECTRICITY EXPLAINED 49 

mosphere is similar to nitrogen, of which chemical 
nitric acid is a liquid form, and for this reason Hert- 
zian waves can be detected jumping through the acid 
between the platinum points of the detector. 

Marconi found that when the primary winding of 
an induction coil was wound on a hollow bobbin and a 
permanently magnetized core was made to shove back 
and forth like a piston rod of an engine within this 
bobbin, and one end of the winding was connected to 
the ground and the other end with an aerial wire, 
Hertizan waves from the air were induced in the 
secondary coil wound over the primary, which were 
detected by strapping a telephone receiver across the 
secondary winding. He then improved this with a 
patent calling for a flexible soft iron wire core form- 
ing a loop of the endless type. This was made to run 
around two pulley wheels through the hollow bobbin 
by a slow-running motor. Around the outside of the 
induction coil were several strong horseshoe magnets 
with heavy lines of force penetrating into the center 
of the coil, and cut by the moving core. This is the 
type of receiving apparatus generally used by the 
Marconi company. 

Lee De Forest has improved this moving-core 
scheme by a patent calling for a soft iron rod, which 
is made to revolve within the hollow bobbin of a 
small induction coil, at either end of which coil is 
placed a horseshoe magnet with its lines of force pene- 
trating into the ends of the windings. This revolv- 
ing core extends out from the coil with a pulley wheel 
fitted on one end, which is revolved by a motor. In 



50 ELECTRICITY EXPLAINED 

so revolving, this core cuts the lines of force from 
the horseshoe magnets, and the electric oscillating 
currents surging through the primary winding will 
be intensified and readily heard through a telephone 
receiver connected with each end of the secondary 
coil. Another form of a De Forest magnetic detector 
is in the shape of a ring type induction coil. The 
primary is wound over an iron core of the endless 
type, with the ends connected as in all cases to the 
ground and to the aerial wire respectively, and the 
secondary coil is wound over this, around the ring. A 
permanently magnetized horseshoe magnet is sus- 
pended from a shaft and pulley, above this ring, and 
made to revolve. This also produces a strong mag- 
netic effect upon the detector. 

That the crystals of some metals and minerals may 
be made use of in connection with wireless detectors 
has been proven by the fact that steel and silicon, 
fused into a crystal in an electric furnace and known 
as carborundum, is now used for that purpose. A 
single crystal is clamped between two flat pieces of 
copper, or brass, or carbon, one of these plates con- 
necting with a telephone receiver and the other plate 
connecting with one side of a battery. The other 
side of the telephone receiver connects with a lever 
made to slide over a variable resistance coil, which 
coil connects with the two sides of the battery. This 
variable resistance device is called a potentiometer, 
and is shown connected to the carborundum crystal 
in an accompaning drawing. Loadstone has been used 
in the same way, instead of carborundum, and also 



ELECTRICITY EXPLAINED 



5i 



silicon crystals, to detect wireless messages. A more 
simple way to use any of these crystals is to adjust 
a brass screw close against the crystal used and with- 
out a potentiometer connect the crystal with the 
ground, and the brass screw with an aerial wire. Then 
strap a telephone receiver across these two connecting 




CARBORUNDUM DETECTOR. 

wires. A slight current of electricity is formed by rea- 
son of the oscillating current discharges heating the 
two dissimilar metals, and this slight change of tem- 
perature, producing fluctuating currents, click in the 
receiver in accord with incoming Hertzian waves, 

Now that we have learned of ways and means bv 
which wireless messages may be received we begin to 
consider the necessity of selective telegraphy through 



52 ELECTRICITY EXPLAINED 

space. It is evident that the Hertzian waves move 
in oscillating currents in the air much as gulf streams 
move in the ocean, and when we send up a kite with 
a wire attached, or any form of aerial into the air and 
get into the right stratum of atmosphere, we can pick 
out messages transmitted with a long wave or with 
a short wave, depending upon which current our aerial 
is in. But this is not perfectly reliable, as there is both 
a magnetic warp and an electric woof to a Hertzian 
wave. If it was not for this spreading out of these 
waves a low aerial would not catch wireless messages 
even when located in an open country field. 

With any stationary aerial we must employ tuning 
coils. A tuning coil is a large winding of small bare 
copper wire placed by the receiving detector and con- 
nected by variable contacts between it and the aerial 
wire, which wire must in all cases be bare. With such 
a variable resistance we can pick out different wave 
lengths and get in tune with any wireless station trans- 
mitting. Around this spiral winding of the receiving 
tuning coil the oscillating currents creep, setting up 
inductive resistance in so doing, and by cutting out one 
spiral after another of this coil we can get in perfect 
attunement with any wireless station using transmit- 
ting tuning coils. The transmitting coil is smaller, 
being composed of a few coils of heavy bare copper 
wire wound in a spiral and placed between the spark 
coil and the aerial. 

There is a manner of tuning by employing un- 
damped Hertzian waves, which is best done by al- 
lowing the spark to jump from the oscillating balls 



ELECTRICITY EXPLAINED 53 

through hydrogen gas. Ordinarily when an electric 
wave oscillates from an aerial it dies down and is said 
to damp out. But if a Hertzian wave can be made to 
hold entirely its original form until it reaches a re- 
ceiving aerial it is undamped, and is called a sine 
wave. A sine wave is capable of giving perfect at- 
tunement. A drawing illustrates the difference in 
form of these waves. 



/yiA 




DAMPED HERTZIAN WAVES. 

A leyden jar helps to make wireless telegraphy se- 
lective, and is used at the transmitter in series with a 
tuning coil. A one-gallon leyden jar will emit waves 
over three hundred yards long, with about one million 
oscillations per second. A pint-size leyden jar will 
emit waves twenty yards long with- about fifteen mil- 
lion oscillations per second. With this wide range 
of attunement, depending upon the size of the jars, 
selective telegraphy is possible through space. 




UNDAMPED HERTZIAN WAVE. 

Another way of making wireless telegraphy select- 
ive is by varying the speed of the oscillating current 



54 ELECTRICITY EXPLAINED 

by making a quick or slow interruption of the pri- 
mary circuit. This is more easily done with a coil 
without an armature current interrupter. With a gen- 
erator connected to the primary winding of the in- 
duction coil, producing a two-phase current, pulsa- 
tions amounting to over fifteen hundred per second 
can be applied, to be converted into a high potential 
oscillating current in the secondary coil with a high 
frequency of over eighty billions per second. This 
kind of wave travels faster than waves of sound, and 
is less liable to interruption than any slower wave. 

Still another way of increasing or decreasing electric 
oscillating currents is by oscillation transformers. The 
step-up transformer is used at the transmitting genera- 
tor in place of the induction coil, connected in the same 
way, and in the receiving circuit with the primary con- 
nected to the aerial and to the ground. These are 
made by winding a few turns of heavy insulated 
copper wire over a coil of many turns of fine wire, 
with an air space between. By immersing this trans- 
former in oil and connecting the primary with a high- 
speed generator we have a high-frequency transmitting 
oil transformer. Without the use of oil we have an 
air transformer. 

Choking coils are sometimes used in the battery 
circuit of the receiver to cut off stray oscillating cur- 
rents. These are made by winding fine insulated wire 
on an iron core and then doubling and winding the 
wire back on itself. A rheostat made with this kind 
of resistance makes a good tuning regulator. 

These and many other forms of apparatus are be- 



ELECTRICITY EXPLAINED 55 

ing constantly improved, and one and all will finally 
bring wireless telegraphy within the reach of all who 
desire communication with distant friends and rela- 
tives at a small expense and without much interfer- 
ence. But unless we all understand the code we will 
not be able to communicate intelligently. So in our 
next chapter we will consider the possibility of actually 
talking through the air. 

However, before leaving the subject of dots and 
dashes, we should mention that there is another use to 
which they can be put besides spelling words. This is 
in sending pictures wirelessly to a receiving station 
where, as with a telegraph register a pencil or pen, 
record is made. To send a picture through the air 
the operator transmits dots and spaces for the light 
lines and dashes for heavy shadows, and a cylinder 
kept rotating automatically at the receiving station 
catches corresponding dots and dashes through a co- 
herer and magnet, which draws to itself an arma- 
ture carrying a pencil. This pencil bears down upon 
the rotating cylinder and marks dots and dashes as 
they are transmitted, and finally a full picture of a 
person or landscape is received. An old phonograph 
cylinder wrapped with a piece of paper to receive 
the pencil marks would make a good register. 



56 ELECTRICITY EXPLAINED 



CHAPTER IX. 

WIRELESS TELEPHONY. 

Alexander Graham Bell, the inventor of the tele- 
phone — that simple piece of electrical apparatus which 
has brought the voices of far-separated friends to- 
gether — no sooner made this peculiar talking machine 
practical than he devised a scheme for talking without 
wires on a beam of sunlight. A metal called selenium 
was found to be an electrical conductor which varied 
its resistance in proportion to the amount of light fall- 
ing upon it. This metal is used in the transmission 
of light and shadow pictures of people over wires, 
and may in the near future bring face to face friends 
separated by many hundreds of miles. Professor Bell 
placed a piece of this metal in front of a large con- 
cave mirror, connecting it in circuit with a telephone 
receiver and battery. Then for the talking appara- 
tus he fastened a small concave mirror to the dia- 
phragm of a transmitter, with the reflecting surface 
facing out toward the distant receiver. In front of 
this concave mirror were placed two separate lenses, 
one a projecting lens and the other a condensing lens. 
With a flat adjustable mirror the sun's rays were 
thrown on the condensing mirror and in focused light 
on to the concave mirror. This small mirror in turn 
threw its beam of light into the projecting lens which 



ELECTRICITY EXPLAINED 



57 



was placed in line with the selenium cell at the re- 
ceiver. When Mr. Bell talked against the diaphragm 
of his transmitter, constantly varying rays of light 
pushed out against the selenium cell, varying the re- 
sistance of this cell accordingly, thereby deflecting the 
receiver diaphragm so that his voice was heard. 





BELL S RADIOPHONE. 

Herr Ernest Ruhmer took up this branch of wire- 
less telephony and devised a speaking arc light. A 
direct current generator of fifty volts furnished the 
arc light which was placed in front of a concave mir- 
ror. Two condensers were placed behind this mirror, 
one side of each connecting respectively with the two 
carbon electrodes of the light. The other two sides of 
the condenser were connected, one to each end of the 
secondary winding of an induction coil. The primary 
winding of this coil was in circuit with a transmitter 
and local battery. With this transmitter Ruhmer was 
able to talk several miles to the selenum cell receiver 
of Bell's radiophone. The voice caused the sound 
waves to vary the resistance of the transmitter, as in 
the ordinary telephone, which set up intermittent cur- 
rents in the secondary coil. The condensers in circuit 
between the generator and the secondary coil prevented 



58 



ELECTRICITY EXPLAINED 



this machine-produced current from backing up into 
the coil. 

There are many inventors working with wireless 
telephony, and various are their ways of working. A. 
F. Collins has a portable wireless telephone which can 
be carried in one's pocket and batteries strapped 
around one's waist. Also as far back as 1898 he de- 
veloped a scheme for talking with wireless waves 
from an induction coil, and in 1902 was using an 
arc-light interrupter with his coil. 





RUHMERS ARCOPHONE. 

Lee De Forest has so successfully made wireless 
telephony practical that his system and apparatus was 
installed on several of the battleships of the Pacific 
fleet. The De Forest Wireless Telegraph Company 
of America has contracted for the erection of an aerial 
on the tower of the Metropolitan Life Insurance 
Building of New York, and preparations are being 
made for wireless conversation from the Eiffel tower 
in Paris to New York City with his apparatus. In 
his system he uses an induction coil with a wire from 
one oscillator resting in an alcohol flame, the pipe fur- 



ELECTRICITY EXPLAINED 59 

nishing alcohol for this flame being connected with 
the ground. By talking into a small megaphone 
against this flame the voice is carried on the Hertzian 
waves. The alcohol flame produces hydrogen, and a 
hydrogen gas in an oscillating current spark gap 
tends to give Hertzian waves shorter length with 
higher frequency, corresponding to the voice waves 
of sound. 

In London there is a Poulson Wireless Telephone 
Company which has patents for burning of hydrogen 
gas in an oscillating current spark gap. Mr. Poulson 
also uses undamped and continuous waves in his wire- 
less conversation, which he has made possible over 
many hundreds of miles. Comparing the damped-out 
wireless telegraph waves and the undamped waves, 
he says : 

'The wireless sparks are to be compared to the shell 
from a big gun. When fired, you get an enormous 
blow like an explosion, but the force of this blow is 
lost after a short time. The undamped waves pro- 
duce a sort of singing vibration of enormous rapidity, 
and they go on their way around the globe with the 
same force as that with which they leave the transmit- 
ting apparatus. Even the highest mountain cannot 
stop them. They do not go through the mountain, but 
they go singing over it until they reach the receiving 
apparatus to which they are appointed." 

The receiving apparatus used in wireless telephon- 
ing may be any style which does not use a filings co- 
herer. The coherer is more adapted to form a local 
circuit to ring a bell or to work a Morse telegraph 



60 ELECTRICITY EXPLAINED 

register or to complete an electric circuit for the sup- 
ply of power to be regulated from a distance without 
wires. 

A Frenchman, Louis Maiche, is the inventor of a 
portable wireless telephone which is claimed to be per- 
fectly selective in its operation. These and many other 
experiments are actively solving the problem of car- 
rying the human voice through the air. 

There is a scheme called the conductivity method 
by which any one can talk without wires for many 
hundred yards. By connecting one side of a telephone 
transmitter to a water pipe and the other side to a 
battery of several dry or liquid cells, which battery 
is connected to a gas pipe on the other side, some one 
can talk into the transmitter, and if we attach a tele- 
phone receiver to two different grounded pipes any- 
where in the same house we will hear the voice at 
the transmitter very audibly. Although the earth in 
which the pipes are imbedded forms a short circuit, 
the resistance is so great that a large portion of the 
waves of electrical transmission is dissipated, and be- 
cause of the cross section of the earth these waves 
flow out in curved lines to the pipes with which the 
receiver is connected. With this method over a dozen 
telephone receivers can be connected on grounded 
pipes and as many people can listen to conversation 
from one central transmitter. 

Another scheme — the inductivity method — is also 
simple in its manner of working, but to make it a 
commercial success will require a great deal of mathe- 
matical solution. We have learned all about an in- 



ELECTRICITY EXPLAINED 61 

duction coil in previous chapters. But perhaps we 
did not stop to think that the secondary coil of wire 
would receive induced currents from the primary coil 
when both were wound on separate iron cores and 
placed at a distance from each other. This is possi- 
ble, and conversation has been carried on in this way 
over one hundred feet. But electromagnetic induc- 
tion varies inversely as the square of the distance, and 
unless we can do away with the square and make our 
coils to vary the induction inversely as the distance 
we cannot hope to talk very far through the air with- 
out great expense. 





-&BBB&S0Z&P 



WIRELESS TELEPHONE. 

It is not necessary to have the transmitting coil 
wound with heavier wire than the receiving coil, and 
for a simple experiment we can use two draw-spark 
coils such as are used in electric gas lighting. With 
a coil made of heavy wire, but having enough of it to 
make a resistance, one end can be connected to a house 
lighting direct current, and the other end to a trans- 
mitter, the other side of the transmitter being con- 
nected to the electric lighting current. A carbon 
transmitter has a resistance of about thirty ohms, but 



62 ELECTRICITY EXPLAINED 

will grow hot and be rendered useless if the coil has 
no resistance to the current. Near this is placed a 
similar coil with its two ends connected to the two 
wires or binding posts of a telephone receiver. When 
some one talks into the transmitter the varying lines 
of force around the transmitting coil will spread out 
into the receiving coil, which coil can then be moved 
away for some distance. It will be found that if 
the coils are not held with their cores parallel electro- 
magnetic induction will not take place, and this prin- 
ciple will give selective telephony. This experiment 
may best be made between two rooms separated by a 
sound-proof wall, as the voice transmitting can then 
only be heard in the telephone receiver. 

The accompanying diagram shows how two or more 
coils may be used both as receiving and as transmit- 
ting coils, depending upon the position of the re- 
ceiver hook. When the receiver is on the hook there 
is a closed path through the coils of wire connecting 
with it, because the hook is made to rest on a contact 
plate. When conversation is desired the receiver is 
taken off the hook and the battery and transmitter are 
thrown in circuit with the coils by the hook resting 
against another contact plate. This receiver hook of 
course must be forced against the upper contact plate 
by a stiff spring, as are all telephone hooks, the weight 
of the receiver serving to hold it down when on the 
hook. The diagram represents a battery supply of 
current, but it is perfectly safe to use a strong direct 
current installed in the house for lighting, and more 
simple, as it does away with cells that are not lasting. 



ELECTRICITY EXPLAINED 63 

With a plug screwed into a lamp socket and the trans- 
mitter and induction coils connected in series with the 
two wires leading from this plug when the receiver 
is off the hook, one can talk over the air without any 
grounded connection whatever if the party listening 
to the message holds the receiver hook down at the 
receiving apparatus. 

If a battery of two or more dry cells are connected 
in circuit with the receiver and the coils of wire the 
received message will be much plainer, and the electro- 
magnetic induction will vary in proportion to the dis- 
tance. Or we can accomplish this by inserting in the 
middle of the stranded iron core of each coil a perma- 
nent bar magnet. In either case we throw out lines 
of force around the receiving coils which stretch out 
and mingle with those encircling the transmitting coils, 
whereas before there were no lines of force around 
the receiving coils to catch the electromagnetic induc- 
tion. 



64 ELECTRICITY EXPLAINED 

APPENDIX OF FORMULAE. 

C = E-^R 

E = CXR 

R = Eh-C 

Q==CXS 

F = Q-^-E 

W = EXC 

C means Current or Amperes. 

E means Electromotive Force or Volts. 

R means Resistance or Ohms. 

Q means Quantity or Coulomb. 

F means Farad or Capacity. 

S means per Second or Seconds. 

W means Watt or Power. 



OCT 2 1909 



